A box presents a plurality of suction orifices, some of which act as gripping points only if they are covered by the article for handling. Unfortunately, numerous orifices are left uncovered by an article, and as a result the load on the vacuum generator is considerably increased.
With suction cups, this gripping function is not provided in the event of the suction cup failing or in the absence of an article beneath it.
In order to avoid those drawbacks, each suction duct or orifice is provided in conventional manner with a shutter or closure device that automatically interrupts the connection between the orifice and the suction box. More precisely, in a box, it is known to provide a chamber in the box wall and to place within the chamber a valve, e.g. having a moving member in the form of a ball, which valve member is free to move within certain limits. The chamber is made to be large enough so that, during normal operation, the vacuum created in the orifice over an underlying article does not raise the ball. If no article is present in register with the orifice, air is sucked in and the flow through the chamber is sufficient to entrain the ball and lift it to the valve seat via which the chamber is connected to the inside volume of the box, so as to press the ball in leaktight manner against its orifice, thereby taking the orifice out of service. For this purpose, it is appropriate for the stroke that is available to the ball, for the weight of the ball, and for the dimensions of the orifice to be determined appropriately relative to one another.
It is also necessary to make provision for this isolation of the orifice from the atmosphere to cease when a part becomes present in register with the orifice that had previously been taken out of action by the valve ball. For this purpose, an air channel of calibrated section is provided in parallel with the valve chamber. Thus, in the presence of an object that closes off the orifice in leaktight manner together with the surrounding zone into which the calibrated duct opens out, it is possible to cause the pressure in said leaktight space to drop and thus to lower the pressure in the chamber upstream from the valve member. When the pressure becomes low enough, the ball separates from its seat and a larger section is thus opened for suction. This improves taking hold of the article and retention thereof.
Document EP 1 012 101 describes a device of the above type. The machining required to fabricate it is expensive since it requires two parallel channels to be provided, one of which is of an extremely precise diameter so that the permanent leak it establishes between the vacuum source and the atmosphere is under control and does not overload the vacuum generator.